I don't mean to be rude, but what is the basis for saying that puffers grow fast? I have have kept many small dogface puffers in 55 gallon tanks and in my experience they grow really slow. The trigger might be an issue. I had a small Niger that grew 3" in one year and they don't just get long, they grow up and out too!
Personally, I don't see a problem with putting a dogface in there. Try and find a small striped dogface (sometimes fall under different common names). Just remember that in about a year, he's going to need a new home. I have a friend that has a Yellow Dogface thats been in captivity for about 5 years (it was good size when she purchased it), that is bigger than a football, before he puffs up!
Another option is a small spotted puffer. It's a "brackish" fish that is commonly converted to salt.
You know what your doing. Just be responsible with what you have. That's what were all trying to do.
